Shower circuit 10mm

[ultinator]

Was reading an older thread and dont have any books to hand, but basically.

If a cable for a 10kw shower is to be run, the current drawn would be 43.4 A.

So if 6mm cable was appropriate for say 70% of the run that was clipped direct, but not suitable for 30% of the run due to de-rating factors, as it would be covered by insulation . Could you size up to a 10mm cable just before the cable hit the insulation part of the run, the drop back to 6mm after the insulation where it may be clipped direct again.

I would like to note that this a hypothetical question, and the thought behind it was that if you had a long run then being able to buy only 30% of the run in 10mm would save you monies on cables.

Also (on a separate note) with gu10 firerated lights is there a recommended clearance of insulation from around them i.e. in a loft where the lights come up from ceiling below, as I presume you would want as much insulation to remain as possible.

Hope that all makes sense, thanks.

 

[huntos33]

Lamps with aluminum reflectors help with the 2nd issue by deflecting all the heat from the lamp downwards, normally i just push the insulation aside to 6" or so , to be honest im not sure if there is a recommended distance across the board

 

[sparks1973]

and for how much (length) is the cable run in insulation?

 

[sparks1973]

Table F2 OSG states that derating for insulation for 500mm and over is 0.50....so you have a 10KW shower...the design current (Ib) for this will be 43.47A...so the current carrying capacity (Iz) for the cable has to meet this AFTER any derating is carried out....

 

[sparks1973]

look, if a portion of the run is ref. method 100 or method A......then why not just run the whole lot in 10mm?....

 

[ultinator]

thanks for the swift replies,

to be honest I'm not really after the calcs and specifics just merely curious as to whether the derating factors only apply to specific parts of a run and if so is it acceptable to use larger cable for just this part, so you could have a JB or isolator switch to go from the smaller cable to the larger cable prior to the length of run within the derating area, for lack of a better term (which would be suitable after applying the correction factors in the given conditions)

If this is still unclear, im picturing 45A breaker, 6mm cable under floor boards, rise up beneath stairs, into airing cupboard, up into loft, across insulation (at which point im asking if, if this part of run would derate the 6mm then could you run 10mm until your away from insulation) down into 45A DP shower pull cord, then cable back up and dropped down stud wall cavity for 10kw shower...

 

[Richard Burns]

Was reading an older thread and dont have any books to hand, but basically.

If a cable for a 10kw shower is to be run, the current drawn would be 43.4 A.

So if 6mm cable was appropriate for say 70% of the run that was clipped direct, but not suitable for 30% of the run due to de-rating factors, as it would be covered by insulation . Could you size up to a 10mm cable just before the cable hit the insulation part of the run, the drop back to 6mm after the insulation where it may be clipped direct again.

I would like to note that this a hypothetical question, and the thought behind it was that if you had a long run then being able to buy only 30% of the run in 10mm would save you monies on cables.

You could do this and meet the regs but the cost of buying the junction boxes /crimps and the time taken to connect the cables would likely outweigh the additional cost of the cable.

Also (on a separate note) with gu10 firerated lights is there a recommended clearance of insulation from around them i.e. in a loft where the lights come up from ceiling below, as I presume you would want as much insulation to remain as possible.
Follow the manufacturers instructions, they vary widely.

Hope that all makes sense, thanks.

See the notes above.

 

[ultinator]

look, if a portion of the run is ref. method 100 or method A......then why not just run the whole lot in 10mm?....

I am aiming to hypothesise a situation that may never arise, but point being, if the run is 50m and only 10m is through insulation and needs 10mm, and the other 40m can be run in 6mm then why spend all that extra money to get 50m of 10mm as im sure 10m of 10mm and 40m of 6mm is cheaper that 50m of 50mm

I think I have made a simple question quite confusing....

 

[ultinator]

Also sorry for alot of these posts as they keep springing up whilst im writing something

 

[sparks1973]

well i wouldn`t encourage unnecessary joints in cable/s for a start...

 

[ultinator]

50m 10mm T+E £170

10m 10mm T+E £35
50m 6mm T+E £ 75

This is just a question i wondered to myself, and so the length of runs could vary but as long as it conformed to 7671 and the joint in cable was necessary due to derating, then on a '50m' run you can save £60 or 1/3 of the cost of cable by this method. I would again like to stress I am not contemplating this as an installation I was just wondering and thought someone may be able to satisfy my curiosity.

 

[sparks1973]

well as long as joints were accessable for inspection/maintainence....but i wouldn`t do it....just pull a 10mm in....

- - - Updated - - -

you would price accordingly....and if the client wont see it.....

 

[ipf]

Have you thought about volt drop. If the run is 50 metres.....work things out.

 

[drew35]

Isn't there a reg about stepping up cable size? Thumbing my big green one now!!!

 

[Engineer54]

Isn't there a reg about stepping up cable size? Thumbing my big green one now!!!

Think your referring to stepping down a cable size!! The bigger the cable the better, except for your, or your clients bank balance!! lol!!!

Getting back to the topic, yes you can do it this way, but no self respecting spark would do so, your just adding unnecessary joints and weakness into a circuit. On top of that, it's going to probably work out cheaper to run the circuit as it should be done, in the lager cable size, when considering the overall costs...